Device for the transmission of signals by means of frequency-modulated carrier waves



Dec. 14, 1948. J. VAN DER MARK ETAL 2,455,959

DEVICE FOR THE TRANSMISSION OF SIGNALS BY MEANS OF FREQUENCY-MODULATED CARRIER WAVES Filed March 24, 1943 gnaw-.115 fly I fre uencies iri cycles per second INVENTORQ JAN VAN DER MARK q THEODORU-S J. WEYERS ATTORNEY Patented Dec. 14, 1948 DEVICE FOR THE TRANSMISSI NALS BY MEANS F: REQUEN C-.Mopn-- LATED CARRIER WAvEs H f Mark and Theodoi'us Josephus" 9 Netherlands, assignors to- HartfordNationalfBank andLTrustICompanm1w. Hartford, Conn, as trustee," ApplicationMai ch 2 1943; sena1 -r-1 ..,4s0',s 62 V f Inthe'Netherlands Jan van diet I Weyers, Ei'ndhoven,

2,.Claims.

'Ihis-invention relates to a device for the transmission of signals by means of a frequency-modue lat-ed carrier wave. It is known respects the use of tions may have great modulated oscillations,

that in various frequency-modulated oscilla- I advantages over amplitude:

The invention has for its object'to-add to these advantages another advantage viz; a considerable decrease of the distortions which may arise when the emitted signal reaches the 'receiVer'al'ong dl-fferentpaths; V

The invention is based 'onthe recognition that upon reception of frequency-modulated oscillations reaching the receiver along two paths with a difference in transit time T the, distortion occurring after detection depends on the frequency and is approximately inversely propo'rtional to thefrequency of the modulating oscilla-' tions. Consequently, the distortion i'stroublesome more particularly at the lower frequencies andfor this reason a device 'for the'transmission offrequency-modulated to the invention, arrangedso that at 'leastapart of" the frequency spectrum insuch manner that the frequency spectrum of oscillations is, according v i of the signals to be" transmitted is heterodyned before the modulator the oscillations modulating the carrier wave to be I transmitted solely comprises frequencies above a definite frequency whichexceeds thelowest fre-' quency of the signal frequencies to betransmitted. i I

The" frequency spectrum of the signals to be transmitted is preferably shifted towards the higher frequencies by an amount corresponding at least substantially tothe r-nini'murnfrequency which may still appearin the oscillatidnsmodulating the carrier wave and which preferably amounts-to about 1 000 cycles/sec; v

The-choice of-the minimum frequency is deter m ned on the one hand by the admissible-distortion which occurs after detection due totransmission along two different paths, but on the other hand the minimum'frequency should not be chosen too high, since otherwise there occur toohigh modulation frequencies"and, as is wellknown, theratio between signal and the dis turbances caused'in a receiving set byother transmitters; by noise" or by atmospheri'cs' becomes worse as the ratio between the'frequencydevia tion and the highest modulation frequency de creases f The invention willbemore fully explained-by reference to: the accompanying "drawing: 'given one form of e construction of a transmittingdevi'ce by way of example; in whichle 'represents Vcarrie'r wave to be :rang em'ent for the-reception of frequency mod e cycles/sec, transmitted by 'the filt er' *F'z. this I or? sla are? 119 according" to the invention,- in which that part of the frequency spectrum of the signals to betranj smitted, which lies below the lowest admissible frequency of the liscillatio'ns' modulating the transmitted is heterodyned in transformed spectrum does' f such manner that the k I "th ljj's' lier e not comprise frequencies below quen'cy. 2 v I A *Fi 16 schematically represents a receiving a'r lated oscillations transmittedby means of sfdeofthe sig'nal'sto be amount corresponding to emitted by means of a device as shown in-Fig. 2a. In describing the executional examples illustrated in thedrawing it-is postulatedthat'the'fre quency" spectrum of the signal'si to be transmitted comprises a'frequency' range of'0 to'10;000 cycl sec. and that the oscillations modulating th'e c 1 rier-wave may" solely: comprisefrequenci'es above 1000cycles/sec. I

' the form. of construction represented in- Fi la; thessignalsi to be transmitted;:lwhi'cht=are generated: in. amicrophone' Q; are supplied through: a lowfrequencyuamplifier. LE1 toxtwo filters Fr and off which the, filter F1 transmits the sighah frequencies exceeding'the lowest frequency'which may sun appear in the oscillations modulati'ngth'e carrier" wave, the filter F2 transmitting the signer frequencies which 1 are lower than the said "fr'e quency. The frequencie'saof- 1000 to 10,000fcyc1es/ sec. transmitted by the filter Fr are-"directly 's"1J'.p pliedt'o' a frequency modulator-FM; and the 5 fre quenci'es (if '0 to1 000 cycles/sec}transmitted by" the filter Fg are heterodyned, before being' sup -fplied to thefrequency modulator in' -si.i.cl 1" manner that the frequency spectrum of'th' na formed oscillations nolonger comprises frequen cies below =-1000" cycles/sec. To this end "the: fr quencies transmitted" by the filter Fa are-supplied toxa modulator'M1 having also supplied toitfanf' oscillation generated by an oscillator Q1 whose frequency is higher than the highest magma signal" frequency to be transmitted and amounts to s'aylfighllll cyelesisec which oscillation am plitude-modulated by the frequencies of-0 to 1000* way there are produced=two -side 'bands com "1 prising a frequency range of 10,500 to 11,500 cycles/sec. and 10,500 to 9500 cycles/sec. respec tively. In the modulator M1 the carrier wave of 10,500 cycles/sec. is preferably suppressed, whereas the two side bands are supplied to a filter F3, which transmits only one of the side bands and, for the chosen value of the frequency of the oscillator 01 passes the upper side band of 10,500 to 11,500 cycles/sec. This sid band, whose fre-- quency spectrum includes the heterodyned fre quencies of 0 to 1000 cycles/sec. of the signals to be transmitted, is supplied to the frequencymodulator FM, in which modulator the carrier wave generated by an oscillator 02 is frequencymodulated by the frequencies supplied to the modulator. The frequency spectrum of-the oscillations modulating the carrier wave includes the frequency band of 1000 to 10,000 cycles/secjtransmitted by the filter F1 and the frequency band of 10,500 to 11,500 cycles/sec. transmitted by the filter F3, which frequency bands do not comprise;

frequencies below 1000 cycles/sec.

The oscillations frequency-modulated in the modulator are emitted by an antenna A.

The choice of the frequency of the oscillator 01 determines the width of the frequency spectrum of theoscillations modulating the carrier wave. This frequency is preferably so chosen that the frequency spectrum of the modulating oscillationsis but slightly broader than the frequency spectrum of the signals to be transmitted; In fact, it is advantageous inthe receiver that the frequency spectrum should not be broader than is strictly necessary, since the energy of the disturbances produced in with an increasing width trum of the oscillations frequency-modulating the carrier wave. In fact, the energy of the disturbances occurring is proportional to sents the maximum frequency of the modulating oscillations.

of the frequency spec- With an extension of the frequency spectrum,-

consequently at a higher value of fm, the energy of thedisturbance appearing in the receiver is larger than will be the case upon reception of a frequency-modulated carrier wave; inwhich case the frequency spectrum lations is as broad as the frequency spectrum of the signals to be transmitted.

The receiver represented in Fig. 1b which is adapted for-the reception of the frequency-modulated oscillations emitted by means of adevice as shown in Fig. 1a comprises a high-frequency amplifier HF to which are supplied the oscillatlons picked up by an antenna A1. The amplified oscillations set up in the output circuit of the amplifier-HF are preferably supplied through a frequency transformer or modulator M2, to which are supplied oscillations locally generated by an oscillator 03, to a detector D, in which the fre-- quency variations of the carrier wave are transformed into amplitude variations which are subsequently rectified. The oscillations supplied by the. detector comprise a frequency spectrum which correspondsto the frequency spectrum of the oscillations modulating the incoming carrier wave and extends from 1000 to 10,000 cycles/sec. and from 10,500 to 11,500 cycles/sec. In order to transform this frequency spectrum into the frequency spectrum of the signals to be received,

the receiver increases of the modulating oscilthe oscillations in the output circuit of the detector D are supplied to two filters F3 and F4, of

which the filter F3 transmits the frequency band I of that part of the signals to be transmitted which is transmitted by the filter F2 in the transmitter. For this purpose the frequencies of 10,500 to 11,500 cycles/sec. transmitted by the filter F4 are suppliedto a modulator M3, which has also supplied to it an oscillation which is generated by the oscillator 04 and has a frequency of 10,500 cycles/sec. In the modulator M3 there arise two frequency bands of 21,000 to 22,000 cycles/sec. and of 0 to 1000 cycles/sec. respectively, of which the lower side band including the frequencies Of 0 to 1000 cycles/sec. is transmitted .by a filter F5 and supplied to the low frequency amplifier LFa In the low-frequency amplifier are amplified the frequency band of 1000 to 10,000 cycles/sec. transmitted by the filter F3 and the frequencyband of 0 to 1000 cycles/sec. transmitted by the filter F5, which bands jointly comprise the frequency spectrum of be transmitted, the oscillations set in which the amplified oscillations are trans- ..'formed into the sound vibrations to be received.

Fig. 2a represents one form of construction of a device according to the invention, in which the of the frequency spectrum of the signals to be transmitted. In Fig. 2a the parts corresponding 1 to the arrangement shown in Fig. 1a bear the -;same reference numerals; furthermore, it is assumed again that the frequency spectrum of the signals to be transmitted includes the frequency range of 0 to 10,000 cycles/sec.

In the form of construction represented in Fig: .2a the oscillations generated in the microphone Q, which comprise the frequency spectrum of 0 to 10,000 cycles/sec. are supplied through the amplifier LFI to a modulator M4, which has also supplied to it an auxiliary oscillation supplied by, an oscillator 05 whose frequency at least corresponds to the highest signal frequency to be transmitted plus the lowest frequency which may still appear in the oscillations modulating the carrier wave I to be transmitted and amounts to say 30,000

cycles/sec, which oscillation is amplitude-modu-' lated by the amplified signals to be transmitted.

In this way there arise two side bands comprising a frequency range of 30,000 to 40,000 cycles/sec. and 30,000 to 20,000 cycles/sec. respectively- Preferably the oscillation of 30,000 cycles/sec. is suppressed in the modulator M4, the two side bands being supplied to a filter F6, which solely transmits the upper side band, and supplies it to a modulator M5, which has also supplied to it a second auxiliary oscillation supplied by .an:oscil-,,,;

t e, s s t up in the output circuit being supplied to a loud-speaker LS,

later-t,"whose-frequency is1000cyclelsecnhlghrthan the highest frequency of the upper side-band and consequently has a frequency of 41,000 cycles/sec. In the modulator also, there ensue ilwo side bands, which comprise a frequency range of 71,000 to 81,000 cycles/sec. and of 11,000 to 1000 cycles/sec. respectively. This last-menhoned; d -b nd, whose wfreaue cy spec i shifted by 1000 cycles/sec. towardsthe hi her-fre quencies with respect to the frequency spectrum of the signals to be transmitted and consequently does not comprise frequencies below- 1000' cyclesfised, is. supplied, through. .avfilter Etsolelt tra'nsmiittingqthis.lower side band, 110 a modulator wherein the carrier way-e produced bynthe generator .02 isv frequency-modulated "by these side-band frequencies the described form- 0f construction the frequency speotrum of the; signals to be transmitted isjnot onlyshifted as a whole by 1000 cycleslsec but also thewfrequency spectrum is: reversed,- as, a RQSwtJQf which .the low and highfrequenciesreispectively of the signals tov .be transmitted are and low frequencies respectively in the ire-- quency spectrum of .thev oscillations modulating the carrier wave.

This reversal of the frequency-spectrum does not take place if the auxiliary oscillation supplied by the oscillator 06 has a frequency which is 1000 cycles/sec. lower than 7 frequency band of 30,000 to 453,000 cycles/sec. transmitted by the filter Fe and consequently supplies an oscillation having a frequency of 29,000 cycles/sec. In this case there ensue in the modulator M two side bands comprising a frequency range of 59,000 to 70,000 cycles/sec, and of 1000 to 11,000 cycles/sec. respectively, the frequency spectrum of the lower side band being shifted by an amount of 1000 cycles/sec. with respect to the frequency spectrum of the signals to be transmitted.

However, it is preferable that the oscillator 06 should produce oscillations whose frequency lies beyond the side bands produced in the modulator M4; in order to achieve the desired displacement of the frequency spectrum of the signals to be transmitted the oscillator 05, provided the filter Fe transmits the upper side band of the side bands obtained in the modulator M4 which comprises the frequency range of 30,000 to 41,000 cycles/see, will consequently be tuned to a frequency which exceeds by 1000 cycles/ sec. the highest frequency of this frequency band, but if the filter F6 transmits the lowest side band of 30,000 to 20,000 cycles/sec, the oscillator 06 will be tuned to a frequency which is 1000 cycles/sec. lower than the lowest frequency of the lower side band. In both casesv the desired displacement of the frequency spectrum is accompanied by reversal of the frequency spectrum.

The receiver represented in Fig. 2b, which is adapted for the reception of the frequency-modulated oscillations emitted by means of a device as shown in Fig. 2d, comprises a high-frequency amplifier HF, to which are supplied the oscillations picked up by the antenna A1, the frequency transformer or modulator M2 and. the detector D, the latter transforming frequency variations of the carrier Wave into amplitude variations which are subsequently rectified. The oscillation supplied by the detector comprise afrequency spectrum which corresponds to the frequency spectrum of the oscillations modulating the incoming carrier wave and extends from 11,000 to 1000 cycles/sec. In order to transform this frequency spectrum thelowest frequency of the quency of into the'- frequency spectrumfiofthe signals to be received the oscillations-fin the-output circuit. of: the detector-Dare supplied-itch modulator'Ms. having supplied to'it: an auxiliary oscillation supplied byan oscillator-10a whose frequency amounts to say 41,000-cycles/sec; 1 'fn-theimodulator Ms'the oscillations generated by'the. oscillator .07 are amplitude modulated by the oscillations. compris ing the frequency spectrum of rooo ito 11,000

this Ecase two'side bands ensue again, of which the lower; which includes-ti1e=frequencies of -0 tol0',000 lcycl'es/sec., corresponds to the frequency spectrum- 0f the'signal to be received. This lower side 'bandis supplied through a filter F9, which, stops the carrier waveand-the upper the: low frequencynmplifler LFz and subsequently to-the l'oundspeaker in which the incoming oscih'ations, comprising a frequencyband of 0 to 10,000 cycles fisee, are transformeduagain into the. incoming soundvibrations'. 1 a l In'the. form of constructionirepresented in Fig. 2b the frequency of theioszci'llationsj supplied by the oscillators Os. endow, corresponds to the: frethe oscillations produced in the transmitter by the oscillators O5 and 06 respectively. This, however, is not imperative; in the receiver transformation of the incoming frequency-modulated oscillations into oscillations comprising a frequency spectrum of 0 to 1000 cycles/sec. may also take place by making the oscillators 0'1 and O8 generate oscillations whose frequency is different from the frequency of the auxiliary oscillations in the transmitter. As an alternative the desired transformation may be obtained by tuning the oscillator 01 to a frequency of 31,000 cycles /sec. and the oscillator 08 to a frequency of 20,000 cycles/sec.

However, in the form of construction under view, in which the whole frequency spectrum is shifted by an amount of 1000 cycles/sec. and conversely, the frequency difference between the oscillators O5 and O6 in the transmitter and the oscillators O8 and O: in the receiver is equal to the minimum admissible modulation frequency of 1000 cycles/sec. in the transmitter plus the highest sign-a1 frequency of 10,000 cycles/sec. to be transmitted. In the present numerical example the difference between the oscillators 05, 0e and Os, 0': respectively consequently corresponds to 11,000 cycles/sec.

We claim:

1. In a frequency modulation system, a source of wave energy of carrier wave frequency to be modulated, a source of modulating potentials coupled to said source of wave energy to modulate the frequency of the latter in accordance with the modulating potentials, and means in said coupling for increasing the frequencies of a band of lower frequency modulation potentials by selected equal amounts which exceed the lowest modulation frequency to reduce at a receiving point distortion at said lower frequency modulation potentials due to multipath transmission comprising, filter circuits for separating a band of modulating potentials in the lower frequency range of said modulating potentials from the of the 'tiwo side bandsthus-produced side band, to

remaining modulation potentials, separate paths coupling said filter circuits to said frequency modulator for supplying the modulating potentials to said frequency modulator and hetero-v dyning means in the path supplying said band of lower frequency modulating potentials.

2. In apparatus for receiving wave energy of carrier wave frequency which has been frequency modulated directly by a band of modulated potentials falling within a. band of frequencies in the higher frequency end of the modulation frequency spectrum and by a subcarrier which has been modulated by the remaining modulating potentials found within a band of frequencies in the lower end of the modulating frequency spectrum to reduce at the receiving point distortion at said frequencies in the lower end of the modulating frequency spectrum due to multipath transmission, modulated wave energy amplifying means, a frequency converter including a source of oscillations coupled to said amplifier, a frequency responsive detector coupled to said converter, means for selecting from said detector the modulation components corresponding to the components used to directly modulate said carrier in the transmitter, means for selecting from said detector the modulation components corresponding to the modulated subcarrier used at said transmitter, a demodulator for deriving from said modulated subcarrier the original modulation components in said band in the lower end of the modulation frequency components, and means at said receiver for recombining said modulation components.

The following references are of record in the file of this patent:

10 UNITED STATES PATENTS Number Name Date 1,491,357 Nichols Apr. 22, 1924 1,907,109 Hinton May 2, 1933 2,059,719 Wilson et a1. Nov. 3, 1936 2,111,023 Dixon Mar. 15, 1938 2,132,205 Dickieson Oct. 4, 1938 2,179,182 Hansell Nov. 7, 1939 2,212,240 Lalande et al Aug. 20, 1940 2,233,183 Roder Feb. 25, 1941 2,301,455 Roberts Nov. 10, 1942 2,323,698 Armstrong July 6, 1943 2,389,356 Goldstine Nov. 20, 1945 2,402,059 Craib June 11, 1946 2,407,259 Dickieson Sept. 10, 1946 2,407,308 Lorenzen et al Sept. 10, 1946 2,408,692 Shore Oct. 1, 1946 FOREIGN PATENTS Number 1 Country Date 466,071 Great Britain May 21, 1937 

